Yeah...I agree. Unless you really want the 'bragging rights' to say "I built it myself", go crate motor! The prices are very reasonable (esp. if you can pick it up yourself, and not pay shipping) and you get a warranty. If you make a mistake putting it together and blow it up.......you'll have to start all over again.
You can use the RV motor as a dummy though! Use it to put in the Z, make all the bracketry and fuel lines, fans etc, then order your crate weight.....that way you won't have a bunch of $$$ tied up sitting in your garage while you're building.
Tim